1. Field of the Invention
Embodiments of the invention generally relate to a valve actuator. More particularly, embodiments of the invention relate to a dual piston actuator for actuating gate valves.
2. Description of the Related Art
Various designs of valve actuators exist that operate to open and close valves in a variety of uses. The petroleum industry utilizes these actuators to operate gate valves that incorporate a sliding gate within a valve body to selectively block fluid flow through piping or tubing. Positioning gate valves along the piping or tubing at various locations controls and directs the flow of fluids therethrough.
A general operation of a gate valve includes moving a valve stem extending from the valve body to move a gate axially within the valve body between an open position and a closed position. When the gate is in the open position, fluid may flow unobstructed through the valve body. When the gate is in the closed position, the gate blocks fluid flow through the valve body. Typically, an actuator piston is used to impart the axial movement to the valve stem to move the gate to the open position. The actuator also includes a spring to continuously bias the valve stem into the closed position. Thus, force applied to the actuator piston from either a hydraulic or pneumatic source, depending on the type of actuator, must overcome the bias of the spring to move the gate to the open position. In addition to the spring bias, the actuator must be capable of developing sufficient thrust to overcome any static gate sealing, initial drag forces on the gate, and the fluid pressure within the valve body opposing movement of the gate and the valve stem.
Generally, a high thrust force is required only to break any static gate sealing and overcome the initial drag on the gate, but a lesser force is required to continue movement of the gate through the remaining stroke to the open position. The excessive thrust during the remaining stroke may provide undue stress and/or fatigue on the valve stem. Conventional hydraulic or pneumatic actuators include fixed diameter actuator pistons that are adjustable in the application of thrust by the fluid pressure supplied to the actuator. Adjusting the fluid supply to the actuator during operation adds complexity to the gate valve operation. Further, depending on the size of the gate valve and/or the amount of thrust force needed, larger actuators require larger volumes of operating fluid and thus resources to move the actuator piston and the gate valve through the full stroke.
Therefore, there is a need for a new and improved valve actuator operable to provide a high initial thrust force and a subsequent reduced thrust force for operating a valve.